The present invention generally pertains to filling of the case of a galvanic cell with viscous active ingredients.
In the production of galvanic cells such as round cells and button cells, the means used to precisely fill the cases (i.e., cups or cans) of the cells with active ingredients, including the electrolyte, tend to occupy a great deal of floor space. What is more, this production step is especially labor-intensive, which helps explain the lack of practical alternatives for improving the filling process, thus making it more economical, through the use of machinery adapted to an automated production flow.
Liquid ingredients (electrolyte) can be added by metering pumps, injecting the precise volume of liquid required through hollow needles. Pressing tools (for example, as disclosed in DE-AS 23 26 460) are generally used to fill cells with primarily solid or powdered active materials or depolarizers (e.g., the material is compressed and compacted in the mouthpiece of a metering tool, and is pushed into the cell cup as a plug of material). Still other types of filling devices are used for metering ingredients which, although free-flowing, are primarily gellike in consistency. The present invention is primarily directed toward the dosing of materials of this latter type, including materials such as electrolyte gels or highly viscous zinc pastes used in alkaline round cells or button cells.
To this end, DE-AS 16 71 861 and DE-PS 26 16 732 disclose devices which allow for the simultaneous injection of materials for the positive electrode, the negative electrode and a prethickened electrolyte. For this purpose, the discharge nozzle of the device includes three coaxially arranged hollow mandrels for forming and discharging strands of material. The conveying mechanisms for these devices usually operate responsive to movements of pistons in hollow cylinders. Desired ingredients are delivered to the cylinders through pressure lines fitted with rotary valves for enabling discontinuous feeding. Volumetric metering is accomplished by stops or through regulation of the piston stroke.
However, in practice, it has been found that the metering of zinc powders (particularly those mixed into pastes with alkali hydroxides and gelatinizing agents) by means of piston pumps controlled by rotary valves is inappropriate when low-mercury zinc is used to satisfy stricter environmental regulations. This is because greater frictional resistance with the particles of the zinc material tends to develop within such pastes as compared to zinc powders with a higher mercury content. This increased frictional resistance produces a pressure buildup in the pump casing, ahead of the cross-sectional constriction in the corresponding metering nozzle.
As a result, the solid and liquid in the paste can separate at narrow gaps in the metering device, such as the gaps developed between the rotary disk valve and the valve housing. The liquid is pressed out of the paste, and the solid particles are left behind, which leads to electrolyte depletion and solidification of the paste. This can at times lead to fusion of the zinc particles, resulting finally in a blockage of the rotary disk valve. This effect is particularly prevalent in zinc powders with Hg concentrations under 1%. As a result, conventional piston pumps are often subject to increased wear, o and needed restorations are very expensive due to the need to replace high-grade ceramic parts.